The high cost and uncertain availability of oil as a fuel has resulted in the increased use of solid fuels, and particularly the burning of wood in various heaters such as stoves, space heaters and boiler units, the burning of wood being particularly attractive in areas which are heavily forested. However, even in cases where wood is inexpensive and easily available, wood is not a complete answer to the problems of heating a home. One major drawback in woodburning systems is the fact that wood burns relatively quickly, and it is therefore not practical to bank a wood fire so that it remains hot overnight, or during the daytime when the occupants are away at work. Under these circumstances coal burns a much longer time. It is therefore apparent that it is highly desirable to provide a heater which is able to burn wood or coal interchangeably so that wood can be used as a cheap fuel curing hours when the premises are occupied, and coal can be used either partly or entirely when the fire is to be banked for longer unattended intervals.
However, the burning characteristics of wood and coal are somewhat different. For instance, coal needs about 80% of the air fed to it from beneath the fire with only about 20% of the air fed from above to burn off escaping volatile gases. Conversely, wood requires about half of the air coming from below and about half of the air supplied above the fire to burn efficiently. The difference in air flow can be controlled primarily by the ash which remains on the grate beneath the burning wood or coal. Since coal requires most of the air coming from below, efficient burning of the coal requires periodic shaking of the grate in order to pass the burned ash into the ashpit and facilitate the flow of air to the bottom of the burning coal fire. On the other hand, when burning wood it is desirable not to shake the grate system so that wood ash tends to remain in the grate beneath the wood and restrict the flow of air upwardly through the grate so that a higher percentage of the air, than would be the case in a coal fire, is fed to the fire above the burning wood. Of course, when a coal fire is to be banked for a long period of time, it becomes undesirable to shake the ash since it is not desirable to burn the coal at a high rate when a long lasting fire is then needed. Moreover, it may occasionally become desirable to dump the fire into the ashpit for the purpose of quickly reducing an overheat condition, and therefore the shaker grid on which the fire rests should be movable to a full dumping position so that the coals can fall into the ashpit unimpeded. Since it is often desirable to burn both coal and wood at the same time, a dual grate system is provided herein so that the coal supporting portion of the system can be shaken down without disturbing the bed of ashes beneath the woodburning portion of the system, or so that the coal-burning portion can be dumped into the ashpit if desired.
German Pat. No. 102,377, which was issued Apr. 6, 1898, shows a dual grate and shaker system in which separate shaker members are supported in two different shaker frames, one behind the other, the shakers having downwardly extending arms which can be used to rock the shaker back and forth. However, this system is not capable of fully dumping the coals in the case of an overheat since the stationary portion of the shaker frame has stationary fingers which interleave with the fingers of the movable shaker grid. U.S. Pat. Nos. 501,623 to Long and 1,207,751 to Hansell show other grate assemblies having two different groups of shakers which can be separately actuated to shakedown the fires in different locations.